https://stackoverflow.com/questions/43890/crop-mp3-to-first-30-seconds
ffmpeg -ss 30 -i inputfile.mp3 -acodec copy outputfile.mp3
findと組み合わせる
https://www.xmisao.com/2013/09/01/how-to-use-find-and-xargs.html
find . -name "*.mp3" -type f | xargs -I {} ffmpeg -ss 3 -i {} -acodec copy {}.mp3